In recent years, results of analysis have indicated that oftentimes, when a driver hurriedly steps on the brake pedal because an obstruction has suddenly appeared in the path of his vehicle, i.e., during emergency braking, the stepping force which the driver applies to the brake pedal is relatively small compared to the stepping force required to produce a brake fluid pressure of a magnitude resulting in wheel lock. Consequently, the braking capability of the vehicle is not fully exerted. Thus, it has been proposed to install on a vehicle a device which determines on the basis of the brake pedal depression speed and the rate of increase in the master cylinder pressure whether the braking operation in progress is a normal braking operation or an emergency braking operation, and when it is determined that it is an emergency braking operation, the brake fluid pressure is automatically raised to a magnitude resulting in wheel lock.
Devices of various construction have been proposed for raising the brake fluid pressure to a magnitude that results in wheel lock upon emergency braking. One of these devices, described in Japanese Unexamined Patent Publication No. H7-251733, involves a vacuum type brake booster that is constructed so that its input-output characteristics can be switched by means of a control signal from outside between at least two different characteristics (a characteristic for normal braking and a characteristic for emergency braking). With the input-output characteristics of the vacuum type brake booster being switched from the characteristic for normal braking to the characteristic for emergency braking at times of emergency braking, it is possible to obtain the same result as when the driver steps on the brake pedal strongly.
With the vacuum type brake booster disclosed in Japanese Unexamined Patent Publication No. H7-251733, however, although when the booster is switched to the characteristic for normal braking a good brake pedal operation feeling is obtained because a reaction force is exerted on the input member of the booster by a reaction member in both the outward stroke and the return stroke of the input member, when the booster is switched to the characteristic for emergency braking, in the outward stroke of the input member a reaction is exerted on the input member by the reaction member but in the return stroke of the input member the reaction from the reaction member to the input member is cut off and consequently a good brake pedal operation feeling is not obtained.
A need exists therefore for a vacuum type brake booster for a vehicle in which a good brake pedal operation feeling is obtained both when the booster has been switched to a normal braking operation and when it has been switched to an emergency braking operation.